1. Field of the Invention
The present invention relates to a thermal transfer type printer for reproducing halftone images or color images by sequentially transferring thermally fusible ink having a plurality of densities or a plurality of colors one upon another onto a printing medium. More particularly, the present invention relates to a device for compensating for fluctuation in the amount of ink which is transferred by such a printer.
2. Discussion of the Background
In a printer of the type described, a plurality of kinds of thermally fusible ink provided on an ink sheet are sequentially fused by heat generating elements which are supported by a thermal head and, thereby, transferred to a paper one upon another. Assuming that all the different kinds of ink share the same physical properties such as fusing temperature and specific heat, the energy required for fusing, or transferring, ink sequentially increases from the ink to be transferred first toward the ink to be transferred last. Specifically, an image is synthesized on a recording medium in multiple layers by transferring different kinds of ink such that they lie one upon another in dots on the medium and, since the transfer energy applied to one of the different kinds of ink for transferring it in a dot is absorbed by another kind of ink which has been transferred to the dot before, greater energy is required for transferring ink associated with the second layer than ink associated with the first or lowermost layer, for transferring ink associated with the third layer than ink associated with the second layer, and so on.
For example, energy necessary for ink to be transferred to a given dot area on a printing medium varies with the number of layers previously transferred to the paper; the former increases with the latter. In addition, since ordinary images are not always solid and, especially, the dot area in the case of color images is modulated, the occupation ratio of ink in the lowermost layer constantly fluctuates causing complemenary variation in the energy which is necessary for the transfer of ink.
Despite the situation discussed above, it has been customary to apply the same transfer energy to all the kinds of ink to be transferred to a printing medium. This, however, allows the amount of ink transfer to fluctuate because precedingly transferred ink absorbs heat applied to ink which is transferred next, resulting in poor image reproduction quality.